Digital still cameras, which have rapidly become popular in the recent years, are designed to electrically record images sensed by CCD sensors and the like as digital data on recording media such as memory cards, unlike conventional silver-halide cameras designed to form object images on the surfaces of films and chemically record the images as analog images.
Digital data can be easily processed by computers and easily distributed through a network and the like. The need for digital still cameras capable of easily obtaining such digital images therefore are expected to increase more and more.
On the other hand, digital data can be easily tampered, e.g., synthesized, without leaving any sign. Therefore, a problem may arise in terms of reliability when a sensed digital image is used as an evidence. Such a problem may not arise often as long as general users enjoy photography as a hobby. However, a serious problem arises when photography is required for some business or legal operation like photographs as records in a construction site.
In addition, since digital data can be easily copied/distributed, the copyright on an image cannot be satisfactorily protected.
Under the circumstances, great expectations are placed on digital still cameras capable of improving the reliability of a sensed digital image as an evidence and protecting copyright.
For such a purpose, a technique called “watermark” has been studied.
According to this technique, in digital image/audio data, another information that is not perceived by a human observer is embedded, and only a person who owns a proper qualification or right can extract the embedded information. This makes it possible to improve the reliability of an image as an evidence or protect copyright.
The principle of a watermarking technique will be described below with reference to Japanese Patent Laid-Open No. 10-290359 and FIG. 2 in a case where digital information is image information (for more detailed information, see this reference and Japanese Patent Laid-Open No. 10-150517).
FIG. 2 is a view showing the flow of a procedure for embedding information (embedded information) in image information.
First of all, an original image (digital image data 101 in FIG. 3) is divided into a plurality of blocks each (102 in FIG. 3) consisting of n pixels×m pixels (division processing). Orthogonal transformation such as discrete cosine transformation (DCT) is performed for each block to obtain n×m frequency component matrices (orthogonal transformation processing).
Before information embedding processing, an embedding position indicating a specific position in the frequency component matrices obtained by orthogonal transformation processing at which to-be-embedded information is embedded is determined by random numbers, and a change amount indicating how much the value of the corresponding frequency component is changed is determined. The embedding position and change amount are then acquired/stored as key information.
When to-be-embedded information is embedded, the information need not be embedded in all frequency component matrices in one block and may be embedded across frequency components in a plurality of blocks. In this case, a block group with proper contrast in the image is selected.
By selecting, for example, a low-frequency portion of the frequency component matrices as an embedding position, information can be embedded so as not to be perceived by a human observer. In addition, a difference from the original value of a frequency component matrix can be changed by changing the change amount. This makes it possible to control a deterioration in image quality.
To-be-embedded information is embedded (embedding processing) by changing the values of the frequency component matrices in the respective blocks on the basis of the embedding position and change amount as key information. In addition, images of a plurality of blocks each having n pixels×m pixels is obtained by performing inverse orthogonal transformation for the frequency component matrices of the respective blocks in which the to-be-embedded information is embedded (inverse orthogonal transformation processing). Finally, the images of the plurality of blocks obtained by inverse orthogonal transformation processing are connected to each other to obtain a watermark image in which to-be-embedded information is embedded (reconstruction processing).
FIG. 2 is a view showing the flow of a procedure for embedding watermark information in an image.
An watermark image is broken up into a plurality of blocks each consisting of n pixels×m pixels (segmentation processing). Orthogonal transformation such as discrete cosine transformation (DCT) is performed for each divided block to obtain n×m frequency component matrices (orthogonal transformation processing). In addition, an embedding position and change amount are obtained from the key information used in the information embedding processing, and embedded information is extracted from the frequency component matrices of the respective blocks (extraction processing).
As described above, for example, characteristic features of the watermarking technique are (1) embedded information cannot be extracted without key information used when the information is embedded, (2) embedded information in key information is generated by using random numbers, and hence is variable and difficult to decode, (3) to-be-embedded information can be embedded so as not to be perceived by a human observer by using a specific embedding position, and (4) the degree of deterioration in image quality can be controlled by changing the change amount.
The above description is about the “invisible data embedding” method of embedding data that is invisible to a human observer. In contrast to this, a “visible data embedding” method is also available, in which copyright information or the like is embedded in an original image in a perceivable state so as to dissuade the third party from fraudulently using the image.
The details of a watermarking technique for such visible data are disclosed in U.S. Pat. No. 5,530,759 (Japanese Patent Laid-Open No. 8-241403).
According to a conventional camera having a watermarking function, when the user is to change the image sensing modes (image sensing mode, drive mode, image quality mode, and sensitivity) of the camera in accordance with the image sensing purpose and object, he/she often changes the embedding mode (type mode and image quality mode) of the watermarking function and to-be-embedded information at once. In this case, in the conventional camera, these settings must be adjusted again, requiring cumbersome operation.